So what I have done is done the entire process of designing, printing and distributing wedding invitations, and your process can be any two, or one or all of these things. Keep in mind you are you, and your couple will decide for yourself what colors, style, and the small details, like whether you want to hold a... Halloween themed wedding.

Whatever floats your boat, man.

A very loose sketch of what you need in materials is:

For the SAVE THE DATE cards
" Cardstock, I used 3-1/2" in by 4-7/8" in cards (to fit)
" 4 Bar envelopes (4" by 5" envelopes see, they fit the cards!)

For the ACTUAL INVITATION
" Color-theme matching Paper (your safest bet is good 'ole cardstock. See photos)

This will include:
" Flat Cards (Smaller cards for your directions and whatnot)
" 8.5 by 11 (Letter) Paper, for putting the most of your directions, information & whatnot

" Envelopes (depending on the size of your largest piece of paper folded to your liking)

This is where I used petal envelopes which I put into another envelope see pictures.

" Stickers? For sealing envelopes, and printing an insignia of your liking.

NOTE: Avoid glossy papers, they look nice... sometimes, but are easily scratched, guffed... as a photographer, they're nice to use but not for graphics. Most things look good on matte and luster paper. Also if you can, try semi-clear vellum papers, they serve as good covers for invitations... We will get into that later.

If it isn't obvious enough, I used paper source at www.paper-source.com as my major supply of paper. But there are tons of other ways too. The major benefit of working with a well established paper provider is that they have consistency in colors, thicknesses, and also a good variety of cuts besides standard rectangles.

But if you're fine with average colors (that is, colors that are close to what you are envisioning) go ahead and find paper off of.. ebay, or just paper you can find wherever you find most convenient. Just if you do, try and get all your paper (of the one color you get) specifically from one place, as not to accidentally get a purple from one place, and then get a puce-purple from another. That would not be good.

My photos also have been adjusted as not to give away the full names of the people I made these for (my sister and now brother-in-law) so the areas in which the names should be in, are a little blurry and crappy looking.
Lets continue.

Step 1: Get Your Words Onto Paper, Write Everything You Think You Will Need

Get a temporary list of recipients, gather your friends, and collaborate with them who you think you should invite. Then put them all in EXCEL. Or some sort of spreadsheet, so you can easily compile their addresses and information together on a single column or row.

The key thing here is to ORGANIZE.

Ask yourself these questions:

" Who are you sending these to? Do they need special accommodations?
" What are you going to say on the invitation? Are you going to send it through your parents?
If so, are you going to type in "Mary and Alex request your presence" or "The families of Mary and Alex..." or "The parents of Mary and Alex...?"

" If you haven't already, ask When will you have the wedding?
" What are your dinner options?
" When, How will you have your rehearsal dinner? (The dinner after the rehearsed wedding. We had ours on a boat, the night before the wedding)
" How will you accommodate your guests? Where do you recommend they stay for a hotel if they are coming abroad?
" How do you get to the wedding location? How do you get to anywhere you're telling them to to go?

Plan to make at the least:
" The Save the Date card and envelope
" The Invitation, reply card and envelope
" Directions and hotel information and envelope
" Stickers or a bunch of pens or a Stamp for Addresses

Optional things are:
" A website
" Frilly things to go with an invitation, like ribbon and patterned paper
" Embossed/Embellished Paper (the paper with texture)

Step 2: Other Languages, Accomodations

Check your friend lists, and check if anyone needs instructions, notifications, whatnot in other languages, and how many of these you need.

Mine are in Japanese, since a good half portion came from Japan to come to the wedding.

Step 3: Save the Date

Start with the Save the Date cards.
(Sorry I have no record of our save the date cards, so I included some external images, they're about the same thing)

They don't have to be anything extravagant. You're just letting people know "you better keep this date open or I'll murder you" ... nicely. (im kidding!)

Get some sticker sheets like the one below. This can be color themed or not.
(The color theme we chose was obviously green, purple and white)
Print your return address on this, as many times as you need.

If you buy your sticker sheets from avery, they provide special templates for you on their website.
http://www.avery.com/avery/en_us/Templates-%26-Software
Simply insert the number of your sticker sheet (it's on the front) and they should provide a template for you to use in Microsoft Word.

Write the simple information down:
Obviously you want the date, and which people are getting married
and simply inform them that a formal invitation should come sooner or later.

The reason why you would need a save the date sent is simple: you can't finish these invitations that fast, and you want to let people know as fast as possible that they should not plan for another event on this day as fast as you can, especially if they're from another planet, and need to buy plane/space ship fare as early as possible to get the lowest price.
Also you don't want them to go to any other wedding on that date. It's your freakin' day! ya got it?
good.

Emailed Save the Dates can work too, but are obviously less elegant.
Creating a website specifically for your wedding is effective as well, and can be made to your liking.

Step 4: Invitation Design

Now that you have the information you want to put on your invitation,
Plan the design for your invitation.

What is symbolic to your relationship?
For this specific wedding, the symbol on the top right side is a icon of a butterfly, symbolic to my mothers side of the family, descendants of a samurai colony that was known for its intricate and beautiful art. They were killed because the didn't practice fighting, and practiced art instead (that's pretty funny, despite its... morbidity?) Anyways, yes we are Japanese.

The sakura (cherry blossom) tree in the back is... well a sakura tree, and a pretty background that I thought would go well with a irregular text design. My father was particularly into silk screening so he actually went and made silkscreens for the sakura pattern. Talk about overachieving...

The following designs are from the internet.
If you're not particularly artistic, that's okay! Ask one of your friends, or simply keep it simple.
I would personally avoid flowery, golden and crazy swirls simply because they're overused, and also a little "simple" goes a LOONGG way.

If you can, stick with a maximum of three colors for the entire theme. This doesn't include the black of text printing, or white of white paper, but if black is a major color, like the second external picture I have here, then count that as a color.

KISS. Keep It Simple Stupid
See following designs.

Another great way to create elegant invitations is to use semi-clear vellum. It is a frosted sheet of paper that creates a soft light on your font, making it appear more... angelic... n'stuff.

Thankfully, we already have an instructables on this:
https://www.instructables.com/id/How-to-print-and-assemble-multi-layer-wedding-invi/

Step 5: Invitation Design (cont')

Paper!

Make your own if you have the time:
https://www.instructables.com/id/How-to-Make-and-Recycle-Paper/

Otherwise, here's a quick guide.
Generally you're safe with simple cardstock, 160 to 205 gsm (the thickness of paper) to print on.
Color is up to you, but keep it a single flat color, or a homogeneous mix of things, like kinwashi paper (pictured below)

A good idea is to keep the important directions small, or foldable, so when they come, they can keep it in their wallet/purse for quick pulling out. Obviously a square wont go into a wallet that easily.
Your main invitation piece, like the one I have, is fine as a square, however. They don't need to bring that.

Step 6: Reply Cards

The reply card should be small, and a pre-stamped envelope to send back. Or you can give the option to call and RSVP, or email back, but you still need to provide them with the information they need to send you.

Such as:
" YES, or NO, if they're coming or not
" What they would like to eat for dinner (so provide a menu)
" If they're coming to the rehearsal dinner
" Who they are, and how many of them are coming
" When to Reply by.

Step 7: Envelopes

This is where the themed part of your invitation plays a major part.
Choose your colors and keep them consistent in your font color, paper, ribbons, and whatever else you have.

Stickers play a good role in sealing your envelopes, or if you want to be extra fancy, you could go for a melted wax press!

For an instructable on how to make your own wax seal:
https://www.instructables.com/id/Homemade-wax-seals-part-1/

EDIT:
If you haven't read the comments, javairny mentioned that wax stamps tend to get eaten by the mail machines, so if you do send 'em, it's probably better to consider larger envelopes, thicker-wise.

Step 8: Finally, Put It All Together!

Now you have your:

" Save the Date Card, and Envelope

" Envelope to include the following:

" Invitation piece, Envelope (mine was the four leafed envelope)

" Reply card, Pre-stamped envelope

" Directions, hotel information sheet along with contact information
" Rehearsal and Rehearsal Dinner information, directions

" Return Address stickers and TO address stickers or stamp or written down

Now all you have to do is go get your stamps, and send it along
with love from me to you!

Step 9: All Together Now, All Together Now!

here's photos that represent what it looks like once they've received the invitation!

Be brave, get creative, and don't be afraid to break every single one of my guidelines.
Rules are made to be broken, and if you're particularly a colorful person, then power to you! Make your invitations rainbow colored for gods sake! It's totally up to you.

Most of all, have fun. This is a great project for you and your partner to take on, especially because of all the time you put in, and the more time you do put in, the more wonderful things you should expect from your wedding, and lifelong marriage.

Just one more thing:
After you receive your reply cards, make sure you start a chart and guest list (as if you didn't know that already) and what we did, was color code the separate groups of people that are coming: whether they are friends, family of the bride, family of the groom, family in japan, extended family, college friends, old friends... so on and so forth.

This will help with table placement and planning.

The whole process cost us about $75 for about... 150~175 invitations, which is a good amount cheaper than getting them from some invitation company.

What really was the cost breaker, was that it brought our families together, as we planned and put together invitations, planned location, times, table placements, activities and more. It brought us together which was more valuable than any of the money we saved.

Have fun!